1. Field of the Invention
The present patent application for industrial invention relates to a driving head for telescopic drill rods and to a machine comprising said head.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Several drilling techniques are known in the state of the art; one of them provides for using a rotary drilling tool mounted at the end of a set of telescopic rods and is usually defined as rotary technique.
Rods are driven into rotation by a driving head operatively connected with a machine; by means of suitable driving means the driving head drives said rods into rotation around their axis, in such manner to transmit motion to the tool mounted at the end of the head rod.
According to such technique, an axial thrust must be sometimes transmitted to the tool or rods, in addition to a rotary momentum.
In order to guarantee a suitable axial thrust for a series of different operations, some telescopic rods are provided with a special internal and external rack distinguished by the particular shape of the teeth.
Such a solution is disclosed in the European patent application EP 0 335 059 in the name of the same applicant.
In brief, said telescopic rods comprise automatic coupling racks, wherein the teeth comprise at least one intermediate section extending in orthogonal direction to the axis of rotation and coinciding with the direction of the vector of the force applied in case of axial thrust or traction.
The driving head of the machine adapted to transmit the momentum and axial force to said telescopic rods comprises a frame to which a cylindrical collar is revolvingly associated, being internally provided with driving racks adapted to engage with the driven racks preach rod to transmit both the angular momentum and the axial force (either thrust or traction).
For this reason, the driving racks and the driven racks are complementary and are provided with complementary teeth for mutual engagement.
The Applicant has noted that, in spite of being an excellent solution, such a solution is impaired by some drawbacks.
The driving racks, which are obtained, in one piece with the cylindrical collar in such manner to protrude from its internal surface, are subject to high wear, both for the considerable loads involved, and for the fact that their use is higher than the driven racks.
In spite of being an excellent solution in terms of resistance, such a solution is impaired by the fact that the replacement of the driving racks is a rather long and difficult operation.
As a matter of fact, some material must be provided, for example by means of welding with a suitable template, to restore the correct shape of the teeth. Such an operation is a long and difficult operation.
Moreover, it was noted that teeth repaired in this way are subject to higher wear compared to new teeth, thus causing the early wear of the driven racks of the rods.
After some studies the Applicant has discovered that the latter drawback is caused by the natural welding shrinkage that may alter, in some cases, the profile of the tooth, when the material has deposited.
Because of the level of the existing forces, if the profiles of the driven teeth and the driving teeth do not have a high level of conjugation, high wear is generated between the parts, thus causing not only the dissipation of mechanical energy and the general heating of the driving head, but also the early wear of the driving and/or driven teeth, according to the specific case.
In other words, the technique that is currently used to repair the teeth of racks originates three types of drawbacks:                difficulty to obtain the best profile in terms of engagement;        difficulty in the repair operation;        excessive overheating attributable not so much to the dissipation of mechanical energy, but to the fact that in the repair operation of the tooth by means of welding, some material is melt, with consequent change of the mechanical properties of the cylindrical collar and potential damage of seals and bearings connecting the collar to the frame of the driving head.        
DE 40 38 424 discloses a Kelly borehole drilling arrangement provided with a rotating drill head coupled with telescopic pipes. The outermost Kelly pipe extends below the drill head and remains in rotary connection with the transmission piece.
The purpose of the present invention is to provide a driving head for telescopic rods able to solve the problems of the prior art.
These purposes are achieved by the present invention according to the first attached claim.